Endless conveyor apparatus are typically used in the mining of coal and the like and incorporate chain/flight conveyors for moving the mined materials from one location to another. Because of the hostile operating environment and the bulky nature of the mined materials, such conveyor apparatus are constructed with very heavy duty materials including heavy gauge conveyor chains and flight bars. Even under the best of conditions, the various working components required to move and support the conveyor chain and flight bars, such as the drive sprockets, support shafts and bearings, are subject to considerable wear. This is due in part to the severe tension created by the weight of the chain and flights, and the forces needed to move the heavy aggregate loads.
Proper tensioning of an endless conveyor during operation is an important consideration for smooth and efficient operation, as well as minimizing wear of the components. It has long been recognized that slack in the conveyor is not desirable since it results in erratic movement and undesirable shock loading of the components, which exacerbates the wear problem. Also, under slack conditions, the chain tends to slip toward the tips of the sprocket teeth, and under severe conditions completely off the sprocket, resulting in immediate equipment failure, and losses due to equipment down time. On the other hand, too much tension in the chain also results in excessive wear of the same components due to the over stressing and strained condition of the apparatus.
One example of addressing the tension problem is found in U.S. Pat. No. 4,090,601 to Freed, which discloses a hydraulic system designed to overcome slack in an articulated conveyor system when lateral movement of a portion of the conveyor is introduced into the system. A hydraulic cylinder, which is responsive to the operating pressure of the drive motor, is operative to maintain only tension sufficient to avoid slack.
Still another U.S. Pat. No. 4,508,213 to Kelley discloses a hydraulic system for the removal of slack in an endless conveyor. The support housings/bearings on the ends of the shaft for the sprocket are shifted longitudinally through a hydraulic cylinder arrangement and a series of linkages. As the housings/bearings move, the shaft and the sprocket are moved to tension the conveyor. The tension is automatically adjusted in response to a pressure transducer. The adjustment, like in Freed '601, is performed only to the extent of maintaining constant tension during normal operation.
Neither of the two references above, or the prior art in general, recognize or address the problem associated with heavy duty conveyors of tensioning the conveyor upon initial operation resulting from at-rest or "breakaway" friction. That is, when starting a heavy duty conveyor, high initial breakaway friction retards or prevents proper movement of the adjustable components when normal operating pressure is used, thus preventing the proper tension being set in the chain or belt. A slack chain results with the attendant undesirable operation. If the pressure, on the other hand, is increased during operation sufficient to accomplish the initial tensioning, undue component stress and wear results from the overtensioning and premature failure of the system results, as described above. Thus, it is desirable to address the conveyor start-up problem associated with breakaway friction, while ensuring that proper operating tension is automatically applied to the conveyor during continued operation.